The present invention relates to a device for generating a reference voltage for a switching circuit including a driving block controlled by an input switching signal and a capacitive bootstrap circuit, in particular for an output stage.
As is known, transistors implemented in MOS technology are often used to manufacture output stage circuits. Such output stages comprise N-channel MOS transistors which are driven by a high supply voltage, the drain of said tansistors being connected to a positive supply line.
It is also known that a problem which occurs with such outut stages is that of ensuring a correct driving voltage for the gate electrode of the MOS tansistors, so as to ensure the operation of the device as a low-resistance switch (with a gate-source voltage V.sub.GS above 10 V).
To this end, a capacitive bootstrap circuit is provided. An example of a prior art applied to a half-bridge is shown in FIG. 1; said circuit comprises capacitive bootstrap circuitry and output stage circuitry, the output stage being driven so as to generate a periodic wave-form signal V.sub.OUT which oscillates between a low voltage, which is about 0 V in the example being considered, and a high voltage substantially equal to a firsst supply voltage V.sub.CC.
In particular, two output transistors T.sub.1 and T.sub.2 are provided in order to generate the periodic wave-form signal and are driven by two driving elements DR.
According to the input signal V.sub.IN, said two transistors T.sub.1 and T.sub.2 are alternately switched in the ON and OFF state so that the output signal switches between the low voltage of 0 V and the high voltage V.sub.CC.
The bootstrap circuitry substantially comprises two elements, i.e. the bootstrap capacitor C.sub.B and the bootstrap diode D.sub.B. When the transistor T.sub.1 is off and the transistor T.sub.2 is on, V.sub.OUT is connected to the ground and therefore is in the LOW state.
In this condition, the bootstrap capacitor C.sub.B is charged through the bootstrap diode D.sub.B at a voltage equal to the difference between a second supply voltage, for example 12 V, and the voltage drop across the bootstrap diode D.sub.B.
In the reverse condition, i.e. with the transistor T.sub.1 in the ON state and the transistor T.sub.2 in the OFF state, the potential of the source electrode of the transistor T.sub.1 rises toward the value of the first reference voltage V.sub.CC, and the bootstrap capacitor maintains the supply to the driving elements DR at about V.sub.CC +12 V.
As can be seen, in this stage the bootstrap diode is reverse, biased, and its reverse biasing voltage is equal to the first supply voltage (V.sub.CC), thus decoupling the bootstrap capacitor from the second supply voltage.
The problem which arises in the above described circuit is that of satisfying two contrasting requirements.
In fact, an output signal having a good dynamic behavior (i.e. considerable amplitude) is required on one hand, while on the other hand it must be possible to integrate the bootstrap diode.
In order to satisfy the first of the above mentioned requirements, the bootstrap diode D.sub.B must have a high breakdown voltage, since, as said, when the output voltage V.sub.OUT is high, the diode D.sub.B has applied thereto a reverse biasing voltage which is equal to the first reference voltage V.sub.CC. This condition is troublesome (in view of the required integration) when the required breakdown voltage of the diode is in the range of hundreds of volts.
On the other hands, in the direct biasing state, the bootstrap diode must have low losses toward the substrate. This is a problem when the diode is implemented by a base to collector junction in order to have high breakdown: in fact in this case the diode is associated with a parasitic vertical PNP transistor.
Therefore the problem arises that it is difficult to integrate a small size diode with a high breakdown and low current losses toward the substrate, and this problem becomes the bigger, the higher is the supply voltage V.sub.CC.